WASHINGTON — The share of Americans without health insurance is dropping to the lowest levels since President Barack Obama took office, but sign-ups under his health care law lag among Hispanics — a big pool of potential beneficiaries.

With just three weeks left to enroll on the new insurance exchanges, the Gallup-Healthways Well-Being Index, finds that 15.9 percent of U.S. adults are uninsured thus far in 2014, down from 17.1 percent for the last three months — or calendar quarter— of 2013.

Released Monday, the survey based on more than 28,000 interviews is a major independent effort to track the health care rollout. The drop of 1.2 percentage points in the uninsured rate translates to about 3 million people gaining coverage.

Gallup said the proportion of Americans who are uninsured is on track to drop to the lowest quarterly level it measured since 2008, before Obama took office.

“It’s probably a reasonable hypothesis that the Affordable Care Act is having something to do with this drop,” said Frank Newport, Gallup’s editor-in-chief. “We saw a continuation of the trend we saw last month; it didn’t bounce back up.”

The survey found that almost every major demographic group made progress getting health insurance, although Hispanics lagged.

With the highest uninsured rate of any racial or ethnic group, Latinos were expected to be major beneficiaries of the new health care law. They are a relatively young population and many are on the lower rungs of the middle class, in jobs that don’t come with health insurance. They’ve also gone big for Obama in his two presidential campaigns.

But the administration’s outreach effort to Hispanics stumbled from the start. The Spanish-language enrollment website, CuidadodeSalud.gov, was delayed due to technical problems. Its name sounds like a clunky translation from English: “Care of Health.”

The feds also translated “Affordable Care Act” as “Law for Care of Health at Low Price” — which doesn’t sound too appealing.

A spot check of the Spanish site on Monday showed parts of it still use a mix of Spanish and English to convey information on such basics as insurance copays, risking confusion.

With disappointing Latino sign-ups, the administration is making a special pitch as the end of open enrollment season approaches March 31.

The president was on Spanish-language television networks last week to raise awareness. Obama assured viewers that signing up for health care won’t trigger the deportation of any relatives who are in the country illegally. The law’s benefits are only for citizens and legal US residents.

Gallup found the biggest drop in the uninsured rate was among households making less than $36,000 a year — a decline of 2.8 percentage points.

Among blacks, the uninsured rate was down by 2.6 percentage points. It declined by 1 percentage point among whites. But Latinos saw a drop of just eight-tenths of a percentage point.

The Gallup poll is considered authoritative because it combines the scope and depth found in government surveys with the timeliness of media sampling. Pollsters interview 500 people a day, 350 days a year. The latest health care results were based on more than 28,000 interviews, or about 28 times as many as in a standard national poll.

The survey can be an early indicator. Gallup saw a modest decline in the uninsured rate in January, and now two full months of data point to a trend emerging.

“This is another indication that the (law) is achieving its key objectives,” said Aaron Albright, a spokesman for the Medicare agency, which is also steering the rollout.

Gallup said the drop coincides with the start of coverage under the health care law on Jan. 1. The major elements are now in effect. Virtually all Americans are required to get covered or risk fines. Insurers can no longer turn away people with health problems. New state-based markets are offering taxpayer-subsidized private insurance to middle-class households.

Medicaid rolls are also growing, with about half the states agreeing to the program expansion in the law. Low-income people who qualify for Medicaid are able to sign up year-round, so the uninsured rate may keep going down even after the end of open enrollment for private coverage.

The administration is citing numbers that are far higher than Gallup’s: about 4 million people signing up for private coverage, and 9 million for Medicaid.

But those statistics also include people who already had health insurance and switched to coverage offered under the law. They also include children, while Gallup focuses on adults.

The survey was based on telephone interviews from Jan. 2-Feb. 28 with a random sample of 28,396 adults aged 18 and older in all 50 states and the District of Columbia. For results based on the total national sample, the margin of error is plus or minus 1 percentage point, larger for subgroups.

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